The present invention relates to a method for separating a target gas from a gas mixture by using a semipermeable membrane. The method of the present invention is suitably applied to a case where a feed amount of a starting gas mixture changes. Furthermore, the method of the present invention is effectively used in a case where a composition of the starting gas mixture fluctuates.
The term "gas mixture" used herein refers to a mixture of a relatively permeable gas ("fast gas") and a relatively impermeable gas ("slow gas"). The term "fast gas" refers to a gas having a relatively high permeability to a semipermeable membrane. The term "slow gas" refers to a gas having a relatively low permeability to the membrane. The "recovering a gas" used herein, in most cases, refers to recovering of the fast gas from the gas mixture at an increased concentration.
When a target constituent gas is recovered from a gas mixture, a semipermeable membrane having a selectivity to the target gas is used. The gas recovery process using a semipermeable membrane has been widely employed in various fields.
Principles of the gas recovery process using the membrane are summarized as follows:
(a) A semipermeable membrane (hereinafter, simply referred to as "membrane") having a selectivity to a specific gas contained in the gas mixture is used for gas-separation.
(b) The feed stream (gas mixture) is fed to one of the sides of the membrane provided within a membrane separation unit, and a permeate stream is recovered from the other side. The side to which the feed stream is fed is called "feed side". The opposite side is called "permeate side". When the feed stream is pressurized and fed to the feed side while maintaining the pressure of the feed side higher than that of the permeate side, the feed stream is separated into a permeate stream which emerges at the permeate side through the membrane and a residue stream which remains on the feed side.
Each of individual constituent gases is driven to permeate through the membrane by a difference between a partial pressure of the feed side (given by multiplying an entire pressure of the feed stream on the feed side by a mole fraction of each constituent gas) and a partial pressure of the permeate side (given by multiplying an entire pressure of the permeate stream on the permeate side by a mole fraction of each constituent gas). The amount of each constituent gas permeated is proportional to a value given by multiplying the difference in the partial pressure by a membrane size (area) and a permeability to the membrane. As a result, the fast gas is enriched in the permeate stream and the slow gas is enriched in the residue stream.
FIG. 5 shows a basic structure of a gas recovering system using a membrane. This system is a single-stage type.
A membrane 1 is provided in a membrane separation unit 2. The membrane separation unit 2 is divided into a feed side and a permeate side by the membrane 1. A feed stream G1 is fed to the system through a feed stream supply line 11, pressurized in a compressor 3, passed through a dryer 4 and a heater 5, and fed to the feed side of the membrane 1 from a feed port of the membrane separation unit 2.
In the membrane separation unit 2, a permeate stream G2 emerging on the permeate side through the membrane 1 passes through a cooler 6 and a permeate stream pressure control valve 22 and is recovered as a product gas G3 from a product gas recovering line 13. On the other hand, a residue stream G4 left on the feed side of the membrane 1 is discharged out of the system from a discharge port of the membrane separation unit 2 via a residue stream pressure control valve 24.
FIG. 6 shows a schematic structure of a gas recovering system of a multi-stage type. The multi-stage type system is widely used for the purpose of increasing a purity and a recovery rate of the product gas.
In this example, two membrane separation units (namely, first membrane separation unit 2a and second membrane separation unit 2b) are used in combination. The feed stream G1 is pressurized in the compressor 3 and fed from a feed port of a first membrane separation unit 2a to the feed side of the first membrane 1a.
The permeate stream G2 emerging on the permeate side through the first membrane 1a passes through the pressure control valve 22 on the permeate side and is recovered from the product gas recovering line 13 as the product gas G3. On the other hand, the residue stream G4 left on the feed side of the first membrane 1a is discharged from a discharge port of the first membrane separation unit 2a and sent to the second membrane separation unit 2b.
The residue stream G4 is fed from a supply port of the second membrane separation unit 2b to a feed side of the second membrane 1b. A permeate stream G5 emerging on the permeate side through the second membrane 1b, passes through a recirculation line 15 and a recirculation pressure control valve 25 and is recirculated to the upstream side of the compressor 3, and then merged into a stream of the feed stream G1. The residue stream G6 left on the feed side of the second separation membrane 1b is discharged out of the system from a discharge port of the second membrane separation unit 2b via the residue stream pressure control valve 26.
The aforementioned multi-stage type gas recovering system is called a "cascade cycle". If necessary, the apparatus may have three or more stages. In the multi-stage gas recovering system, the purity and recovery rate of a product gas can be increased by appropriately setting the size (area) of the membrane of each membrane separation unit and operating conditions (such as operation pressure, temperature) at the time the apparatus is designed, depending upon the feed amount and composition of the feed stream. Furthermore, the multi-stage type is advantageous when the residue stream is also recovered as a product, since the purity and recovery rate of the residue stream can be improved.
The gas recovering process using the aforementioned membrane may be modified depending upon the usage. In some cases, the residue stream as well as the permeate stream is recovered as a product. In other cases, only the residue stream is recovered as a product. Note that the size (area) of the membrane to be used and an operation pressure are determined depending upon a feed stream composition, required specifications and a required recovery rate of the product gas.
In the gas recovering process using the membrane mentioned above, when the feed amount of the feed stream fluctuates while other operating conditions are maintained unchanged, the concentration of a target gas contained in the product gas inevitably fluctuates. This phenomenon is generally undesirable.
For example, if the feed amount of the feed stream is lowered from a reference value while other operating conditions are maintained unchanged, the ratio of the permeate stream increases and the ratio of the residue stream decreases. Furthermore, the fast gas present in the permeate stream increases in recovery rate but decreases in concentration. On the other hand, the slow gas present in the residue stream decreases in recovery rate but increases in concentration.
This case is not preferable if the permeate stream is a target product gas to be recovered, since the concentration of the fast gas present in the product gas is meant to decrease.
In the multi-stage gas recovering system, if the feed stream is fed in a constant amount, it is possible to maintain the concentration and recovery rate of a target gas present in the product gas at the most suitable values. However, if the feed amount of the feed stream is fluctuated, the concentration of the target gas present in the product gas decreases in the same manner as in the single-stage gas recovery process.